1. Field of the Invention
The present invention relates to a knock control system for an internal combustion engine, and more particularly to a knock control system for an internal combustion engine having a plurality of cylinders wherein operation of the engine is switchable between a partial-cylinder operation for operating some of the cylinders and an all-cylinder operation for operating all of the cylinders.
2. Description of the Related Art
A conventional method of determining knocking according to an output signal from a knock sensor mounted on an internal combustion engine for detecting high-frequency vibrations is well-known in the art. For example, Japanese Patent No. 2844418 discloses a determination method, which is discussed below.
In a condition where knocking does not occur, a noise level VNOISE is detected by the knock sensor, and the detected noise level is multiplied by a determination gain GAMP, which is set according to an engine operating condition, to calculate a knocking determination threshold. When the level of a signal indicating an output from the knock sensor exceeds the knocking determination threshold, it is determined that knocking has occurred.
However, if the above-discussed conventional method is used to determine if knocking is occurring in an internal combustion engine having a plurality of cylinders, wherein operation of the engine is switchable between a partial-cylinder operation for operating some of the cylinders and an all-cylinder operation for operating all of the cylinders, the following problems may arise.
For example, it is known that the level of a signal output from the knock sensor tends to become greater during the all-cylinder operation than during the partial-cylinder operation. Accordingly, if the knocking determination threshold is set according to the noise level detected during the all-cylinder operation, the knocking determination threshold is too high to be reliable or effective during the partial-cylinder operation. In other words, it is commonly known that although knocking has occurred, the knocking is not detected during the partial-cylinder operation because the knocking determination threshold has been set too high. Conversely, if the knocking determination threshold is set according to the noise level detected during the partial-cylinder operation, the knocking determination threshold is too low to be reliable or effective during the all-cylinder operation. In other words, even though knocking has not occurred, the conventional method erroneously determines knocking has occurred because the knocking determination threshold is set too low.